1. Field of the Invention
The present invention relates to a self-emission type display apparatus and a driving method thereof, particularly to a technology of correcting burn-in of an organic EL display.
2. Description of the Related Art
Recently, an electronic device using an organic semiconductor material is widely developed, and developments on an organic EL (electroluminescence) that is a light emitting device, an organic TFT (thin film transistor), an organic solar cell, and the like are reported. Among others, the organic EL display is a promising technology closest to practical use. The organic EL display includes a number of organic EL devices, and the number of organic EL devices are disposed in a matrix pattern. In self-emission devices such as the organic EL device, emission luminance of each organic EL device is lowered according to a light emission amount or a light emission time. The lowering of the emission luminance is caused by degradation of emission characteristics. When the degradation of the emission characteristics is progressed, the organic EL device emits the light with different luminance according to the light emission amount or light emission time even if the driving conditions are identical.
In the organic EL display having the above-described emission characteristics, sometimes a phenomenon called “burn-in” is generated when a portion having the large difference in luminance is fixed for a long period of time on the display. When the burn-in is generated, because a burn-in image remains even if another image is displayed, the images overlap each other, thereby posing a problem that the display screen becomes difficult to see.
For example, when movie display is performed, a probability of generating burn-in is low because a same image is seldom continuously displayed for a long period of time at a same place of the display.
However, when a same icon is displayed over a long period of time in a viewfinder of a digital still camera or a digital camcorder, because a same image is always continuously displayed for a long period of time at a same place, the burn-in is easy to generate.
FIGS. 19 to 21 illustrate an example of the burn-in phenomenon. A display 201 of FIG. 19 includes 20 monochrome pixels 202 in the vertical direction by 20 monochrome pixels 202 in the horizontal direction, and a value of each pixel 202 indicates a relative value of emission luminance (hereinafter referred to as luminance value). In the example of FIG. 19, a letter “A” is displayed on the display 201 using the pixels having luminance value of 100. When the letter of FIG. 21 is continuously displayed for a long period of time at the same place, the pixels displayed for the long period of time are degraded, for example, the luminance value is decreased to 90 to become a degraded pixel 302 as illustrated in FIG. 20.
Then, the display 201 including the degraded pixels 302 is changed from the letter “A” to full pixel display in which all the pixels are displayed with the maximum luminance. At this point, as illustrated in FIG. 21, the luminance value becomes 90 only in the degraded pixel 302 used to display the letter “A” while other pixels have the luminance value of 100. Therefore, the portion of the letter “A” becomes adversely dark and the letter “A” is profiled.
In order to keep the burn-in phenomenon inconspicuous, for example, the following technologies have been disclosed.
In a technology disclosed in Japanese Patent Application Laid-Open No. 2002-175041, a cumulative light emission time of each pixel or the cumulative light emission time and emission luminance of each pixel are counted from an original video signal and stored in a memory. Based on correction data previously stored in a correction data memory, a correction circuit corrects the video signal from the cumulative light emission time or the cumulative light emission time and emission luminance, which are stored in the memory, according to a degree of degradation of each self-emission device. Therefore, a luminance variance can be eliminated to obtain a uniform image screen even for a burn-in phenomenon in which the self-emission device is degraded in part of the pixels.
In a technology disclosed in Japanese Patent Application Laid-Open No. 2002-169511, a specific test pattern is displayed when the power is turned on, the luminance is detected by a photoelectric conversion device disposed in each pixel, and the luminance is stored in a storage circuit. Then, a correction circuit corrects an original video signal according to a deficit from a standard luminance (previously stored luminance of a normal self-emission device at the same gradation), and a video image is displayed in the display apparatus.
In the technology disclosed in Japanese Patent Application Laid-Open No. 2002-175041, it is necessary to previously obtain a relationship among the cumulative light emission time of each pixel, a cumulative value of the emission luminance of each pixel, and the degree of degradation of each self-emission device. However, it is difficult to correctly obtain the relationship from only the information on the video signal.
For example, in the organic EL device, a relationship between the emission luminance and an emission luminance half-value time is expressed by Expression 1:
                                          t            1                                t            2                          =                              (                                          L                1                                            L                2                                      )                                -            n                                              (                  Expression          ⁢                                          ⁢          1                )            
In the above expression, L1 and L2 are emission luminances, t1 is an emission luminance half-value time of the emission luminance L1, t2 is an emission luminance half-value time of the emission luminance L2, and n is an acceleration coefficient. That is, the degree of degradation is varied when the emission luminance, that is, the gradation is varied. Furthermore, the acceleration coefficient n is not always kept constant for the emission luminance. Additionally, in consideration to the variation of the degree of degradation due to an environmental temperature or a drive temperature, even if the relationship among the cumulative light emission time, the cumulative value of the emission luminance, and the degree of degradation of each self-emission device is previously obtained, there may be posed a problem that a deviation in degree of degradation is generated between the previously obtained value and each device of the actually used display apparatus.
In the correction technology disclosed in Japanese Patent Application Laid-Open No. 2002-169511, in which the luminance is detected, although the variation of a current-luminance characteristic of the organic EL device can be corrected, it is necessary to prepare a photoelectric conversion device in each pixel in addition to the organic EL device. Accordingly, when the correction technology disclosed in Japanese Patent Application Laid-Open No. 2002-169511 is applied to a high-resolution display, there may be posed a problem that the photoelectric conversion device cannot be disposed in the individual pixel, or an aperture ratio that is a light emitting region is lowered.
Furthermore, because the correction method in which the luminance is detected is influenced by a fluctuation of the characteristic of the photoelectric conversion device in each pixel or a characteristic variation, there is a problem that the burn-in less than the fluctuation of the characteristic of the photoelectric conversion device is hardly corrected.